1. Field of the Invention
The present invention relates to the packaging of electronic assemblies, and more particularly to a construction and method of encapsulating electronic circuits wherein the encapsulating medium is easily removable for reworking or repair of the electronic system.
2. Description of the Prior Art
It is desirable when sensitive electronic circuitry are exposed to an environment of shock and vibration to protect the electronic assembly from damage induced by excessive forces applied to the components. Further, in operation such assemblies generate heat, which must be dissipated. Accordingly, past practice has provided for encapsulating the electronic components to prevent failure due to the effects of mechanical shock and vibration, and further to enhance transfer of heat from the assembly to an external heat sink.
The prior art has utilized rigid epoxies and foamed polyurethanes as encapsulants, with varying degrees of success. When an assembly is encapsulated or potted with epoxy and cured to a rigid state, effective protection to shock and vibration is provided by limiting excessive motion of the circuit boards and electrical components. However, when an assembly is rigidly encapsulated, repairs and circuit modifications are extremely difficult to accomplish, making such repairs frequently uneconomical or permitting damage to the circuit board and components, and resulting consequently in a "throwaway" assembly. The rigid material creates a "brick" which cannot practically be reentered. This prohibits failure analysis and replacement of failed circuits. In addition, the rigid material may cause mechanical failures due to differences in thermal coefficients of expansion between the encapsulant and the circuit elements. High voltage power supplies are particularly sensitive to manufacturing variances, increased capacitances, higher operating losses, and the resultant higher temperatures. DC to DC converters obtain power from an input source and convert it into regulated output power at higher or lower voltages for delivery to a load. Typically, for energizing a cathode ray tube, a voltage of the order of 25 kv may be required. However, not all of the input power is converted to output power, since the supply is not 100% efficient, and some power is dissipated as heat within the converter. Thus, the encapsulant must not hinder heat transfer, and preferably will enhance heat removal from the heat generating components into the surrounding environment. In one program, it was estimated that the cost of scrapping rigidly potted power supplies as opposed to accomplishing repair amount to upwards of $500,000.00 per year.
Another approach has been to encapsulate sensitive assemblies in a substantially rigid, shock absorbing polyurethane foam, which is accomplished by placing the reactive chemicals into an enclosure with the electronic assembly, which reacts to form a foam that fills the spaces between individual components and between the components and the housing. However, the foam has been found to act as an adhesive which bonds the components tightly into the assembly, thus making removal of components for repair very difficult and time consuming.
Liquid insulating fillers, such as oils and fluorinated fluids have been use, but require seals and bellows or other techniques to allow for liquid expansion and may penetrate and adversely affect component performance. Introducing an inert gas or evacuating the assembly housing requires hermetic seals, rigid mechanical component support provisions, and sophisticated means for heat dissipation. It has been found in testing that a ferrite core transformer, rigidly potted as a module, failed when encapsulated in silicone gel or RTV media. While said transformer functioned satisfactorily when encapsulated in a rigid medium, when encapsulated in the gel or RTV the transformer failed due to internal heating, which caused expansion of the transformer and fracture of the module potting. This was attributed to lack of physical support by the compliant media. The problem was exacerbated by poor thermal conductivity of the silicone gel. It was therefore found necessary to increase the thickness of the rigid module potting, which occupied additional space and added weight.
It is clear then that a need exists for an improved encapsulant and method of application which is effective in significantly reducing or eliminating the effects of vibration and shock on circuit components, while facilitating heat dissipation, and yet which may easily be removed from the components to permit them to be tested, serviced, modified, or otherwise processed with ease. A reworkable encapsulant in place of the above described encapsulating media will reduce costs by allowing modules to be reworked instead of scrapped, and cost effective failure analysis would be possible in view of the ease of entry into the module without disturbing the circuit elements. Ease of rework promotes reliability improvements by permitting discovery of the failure modes. Finally, production would be enhanced because elaborate component protection schemes now required with rigid encapsulants would not be required.